The soldier's death was the first reported in Crimea since thousands of Russian troops deployed in the territory shortly after former Ukrainian President Viktor Yanukovych was ousted from power in late February.

In Washington, White House spokesman Jay Carney called the "attempted annexation" of Crimea, signed by Russian President Vladimir Putin in Moscow, a "threat to international peace and security."

He said Washington would impose more sanctions in reaction to the move.

"Those actions have incurred costs already," Carney said. "They have done damage to Russia's economy, to its currency, and to its standing in the world. Further actions, further provocations will lead to higher costs."

On March 17, the European Union and the United States unveiled sanctions targeting some 30 Russians, Ukrainians, and Crimeans.

Carney added that the Russian economy was likely to suffer.

"Russia is taking action that reverses some of the work that that nation had done to establish itself as a responsible leader on the international stage," Carney said. "It isolates Russia, it undermines faith in Russia's commitment to rule of law and therefore undermines the incentive that global investors might have in investing in Russia. That effect has a negative impact on Russia's economy and on the Russian people."

U.S. President Barack Obama and German Chancellor Angela Merkel spoke about the crisis by telephone.

Merkel and Obama also agreed that the March 16 referendum on secession held in Crimea violated Ukraine's constitution and international law and that targeted measures against Russia were a "logical" response.

A statement from the EU’s leadership said “the European Union does not and will not recognize" the annexation of Crimea and Sevastopol.

Russian Foreign Minister Sergei Lavrov called Western sanctions "absolutely unacceptable" and said such measures would have consequences.

The Russian Foreign Ministry said Lavrov delivered the message in a telephone call on March 18 with U.S. Secretary of State John Kerry.

The statement did not give details about any retaliatory measures the Kremlin might take against the United States and European Union.

Speaking to students in Washington later the same day, Kerry chided Russian officials for their interpretation of events.

"I was really struck and somewhat surprised and even disappointed by the interpretations and the facts as they were articulated by [Russian] President [Vladimir Putin]," Kerry said. "With all due respect, they really just didn't jibe with reality or with what's happening on the ground. The president may have his version of history, but I believe that he and Russia for what they have done are on the wrong side of history."

Thousands of Russian forces have been occupying Crimea since late last month.

In his speech to Russian lawmakers earlier in the day, Putin said Crimea's secessionist referendum was democratic and Russia's annexation was in full accordance with international law.

He also dismissed Western criticism as unfair to Russians and Crimeans. Putin cited Kosovo's 2008 unilateral declaration of independence from Serbia, which has been recognized by Western nations but not by Russia.